To produce the conductors on printed circuit boards or, in the case of multi-layered printed circuit boards, on the so-called in-layers, the normally copper-clad epoxy resin glass fibre fabrics are coated with a photoresist or with a similar photo-sensitive and developable polymer layer. The desired structure of conductors is exposed on the dried coating, and the coating is developed at the exposed (or, depending upon the type of photoresist, the unexposed) places. In the etching process which follows, the copper is etched away at the uncovered places, whereas the coated places are protected from the etching effect. When the remainder of the coating has been dissolved away, the in-layer or printed circuit board having the desired structure of conductors is obtained.
The coating and the drying of the in-layers or printed circuit boards are carried out in a coating installation. The latter usually comprises a coating station, in which the in-layers or printed circuit boards are coated on one or both sides with photoresist or the like, and at least one drying station, in which the photoresist is dried and solvent (which may also be water) that evaporates off in the process is transported away from the in-layer or printed circuit board to the outgoing air.
In order to obtain the desired throughput, the in-layers or printed circuit boards have to be dried in a relatively short time. For that purpose, the drying stations are usually in the form of continuous driers in which the coated board-shaped piece material is dried during its transport from an entry opening to an exit opening. The coating is normally dried in a stream of hot air which is passed over the surface of the board. There are also continuous driers in which the coating is dried by means of infrared radiation (IR radiation). As a rule, circulating driers are arranged upstream of those infrared driers in order to evaporate off and transport away any solvent present in the coating. That preliminary drying is absolutely essential in the case of conventional infrared driers on safety grounds since, without it, the solvent vapours which would not be evaporated out and off until the infrared drier was reached could ignite upon contact with the hot surface of the infrared radiators.
An important factor determining the duration of the drying of the coating is the temperature inside the continuous drier. It cannot be set to an arbitrarily high level since that would involve the risk of the solvent vapours' igniting. When infrared driers are used, there is the risk that the coating will form bubbles (boiling bubbles), which severely impairs the evenness and the quality of the coating and may result in an in-layer or a printed circuit board coated in that manner becoming unusable. There are, therefore, limits to shortening the throughput times in conventional known continuous driers.
A further problem with known coating installations and, in particular, also with known conventional driers is the release of solvent vapours into the surroundings. These are able to leave through the entry openings and the exit openings of the individual stations, and adversely affect the surroundings. Just as solvent vapours are released through the entry and exit openings of the stations, however, so it is also possible in the case of known coating installations for contaminants to enter the installation. Those contaminants may settle on the coating and crucially impair the quality thereof. For example, particles in the coating may act as scattering centres in the exposure step or may even mask individual areas with the result that the specified structure of conductors is not completely imaged onto the photoresist. In the processing steps which follow, the copper-clad layer is then etched imperfectly and an unusable printed circuit board is produced.